This application to NHLBI Clinical Trial Pilot Studies describes a pilot clinical study to assess a risk-directed approach to thromboprophylaxis in hospitalized cancer patients at high risk for thrombosis. Venous thromboembolism is a leading cause of preventable in-hospital mortality and hospitalized cancer patients are at a particularly high risk for venous thromboembolic embolic events. The rate of thrombosis in cancer patients often exceeds 10% despite receiving recommended pharmacologic thromboprophylaxis. Cancer patients are also at increased risk of hemorrhage but there is a lack of data supporting the safety pharmacologic thromboprophylaxis in hospitalized cancer patients. We propose a pilot trial in hospitalized cancer patients identified as high risk for thrombosis using a cancer-adjuste prediction model. Patients will be randomized to thromboprophylaxis with standard dose enoxaparin versus intermediate-dose enoxaparin. The primary goals of this pilot trial are: a) to better define the rate of venous thromboembolic events in cancer patients at high risk for thrombosis prescribed standard low molecular weight heparin; b) to assess the rate of major hemorrhage in cancer patients receiving a modified dose of enoxaparin thromboprophylaxis; and c) refine current risk models and the evaluation novel biomarkers such as tissue factor bearing microparticles in order to improve the specificity of risk-directed thromboprophylaxis strategies. These data will enable the conduct of the first randomized phase III clinical trial designed to evaluate the efficacy and safety of thromboprophylaxis in hospitalized cancer patients.